Title: Low cation coordination in oxide melts

Abstract

The complete set of Faber-Ziman partial pair distribution functions for a rare earth oxide liquid were measured for the first time by combining aerodynamic levitation, neutron diffraction, high energy x-ray diffraction and isomorphic substitution using Y2 O3 and Ho2 O3 melts. The average Y- O coordination is measured to be 5.5(2), which is significantly less than the octahedral coordination of crystalline Y2 O3 (or Ho2 O3 ). Investigation of high temperature La2 O3 , ZrO2 , SiO2 , and Al2 O3 melts by x-ray diffraction and molecular dynamics simulations also show lower-than-crystal cation- oxygen coordination. These measurements suggest a general trend towards lower M-O coordination compared to their crystalline counterparts. It is found that this coordination number drop is larger for lower field strength, larger radius cations and is negligible for high field strength (network forming) cations. These findings have broad implications for predicting the local structure and related physical properties of metal-oxide melts and oxide glasses.

The coordination environment of Ti(IV) in a variety of Ti-bearing crystalline oxide and silicate model compounds has been studied using Ti K-edge x-ray absorption fine structure (XAFS) spectroscopy at ambient temperature and pressure in order to provide a quantitative basis for interpreting Ti K-edge XAFS spectra of silicate glasses and melts. Pre-edge features of Ti K-edge XAFS spectra can be used to derive accurate information on the local coordination environment of Ti only if both pre-edge position and heights are considered. Using these features, it is also possible to distinguish between one coordination environment vs. a mixture of several othersmore » (e.g., {sup [4]}Ti vs. {sup [5]}Ti + {sup [6]}Ti). Quantitative analysis of the Ti x-ray absorption near edge structure (XANES) spectra, based on ab-initio multiple-scattering calculations for a variety of Ti-containing clusters and anharmonic analysis of the normalized XAFS oscillations, show that O first neighbors and alkali/alkaline-earth second neighbors around Ti contribute to the XAFS spectra. However, second neighbors are more prominent in the XANES region because the effects of disorder associated with these contributions are less important in this region than in the extended XAFS (EXAFS) region. Therefore, XANES spectra can be used to probe the degree of disorder in the medium-range structural environment around Ti in crystalline and amorphous materials, including Ti-bearing a periodic structures, such as metamict, glassy, and molten compounds. 125 refs., 10 figs., 2 tabs.« less

The local structural environment of Ti in five Na-, K-, and Ca-titanosilicate glass/melts with TiO{sub 2} concentrations ranging from 2.7-30.5 wt% has been determined by in situ Ti K-edge x-ray absorption fine structure (XAFS) spectroscopy at temperatures ranging from 293-1650 K. In parallel, two Ti-model compounds (Ni{sub 2.6}Ti{sub 0.7}O{sub 4} spinel and TiO{sub 2} rutile) were studied under the same conditions to better understand the effects of temperature (anharmonicity) on the XAFS spectra. Temperature-induced anharmonicity was found to vary, largely as a function of the Ti-coordination, and increases significantly around Ti with increasing temperature when present as {sup [6]}Ti. Inmore » contrast, anharmonicity appears negligible around {sup [4]}Ti at temperatures below 1200 K. We predict that anharmonicity should be weak around {sup [5]}Ti as well. No clear evidence was found for a significant change in the average nearest-neighbor coordination environment of Ti in the Na- and K-titanosilicate glasses and melts that exhibit anomalous heat capacities variations just above their glass transition temperatures, T{sub g} (860-930 K). The small (predicted and measured) linear thermal expansion of the ({sup [5]}TiO{sup 2+})--O bond in these systems at high temperature is expected to have an insignificant effect on the local environment of {sup [5]}Ti during the glass-to-supercooled liquid transition. In the most dilute Ti-glass studied (KSl; 2.7 wt% TiO{sub 2}), the local environment around {sup [4]}Ti (especially the second-neighbor alkalis) is relatively ordered at ambient temperature, but this order decreases dramatically above T{sub g}. 41 refs., 11 figs., 3 tabs.« less